OFDMA is a multi-user version of OFDM in which network users are allocated subsets of orthogonal subchannels. Each orthogonal subchannel has an equal bandwidth and is centered on the unique frequency of a subcarrier signal. The frequencies of the subcarrier signals are equally and minimally spaced so data modulation of the subcarrier signals facilitates optimal bandwidth efficiency. A subchannel subset may include all available subchannels or a group of adjacent or non-adjacent subchannels. Subchannel allocation to network users is typically the task of a scheduler. The scheduler may allocate subchannels to users based on criteria such as subchannel quality and users required quality of service.
FIG. 1A illustrates an exemplary frame structure for an IEEE 802.16 OFDMA-based system that operates in a Time Division Duplexing (TDD) mode. Referring to FIG. 1A it is seen that the OFDMA frame consists of a downlink (DL) and an uplink (UL) subframe with the DL subframe 101 always preceding the UL subframe 102. The DL and UL subframes are separated by a Transmit Transition Gap (TTG) 103 and a Receive Transition Gap (RTG) 104. These two gaps prevent DL and UL transmission collisions. Basic frame elements from which dynamic and complex IEEE 802.16 OFDMA frame structures are built are described in the remaining paragraphs of this section.
OFDMA Symbol. An OFDMA symbol is the smallest frame allocation unit in the time domain. In FIG. 1A there are L downlink OFDMA symbols and M uplink OFDMA symbols.
Logical Subchannels. FIG. 1A also shows S logical subchannels for the exemplary IEEE 802.16 DL and UL subframes. Logical subchannels are the smallest logical unit within the frequency domain. A logical subchannel is comprised of a set of physical subchannels that may be adjacent or non-adjacent. The set of subcarriers used for a logical subchannel may change from one OFDMA symbol to another. In a DL subframe, a logical subchannel may be intended for one or more receivers; in an UL subframe, a transmitter may be assigned one or more logical subchannels, and several transmitters may transmit their UL logical subchannels simultaneously.
Logical Subchannel Group. A logical subchannel group is comprised of one or more logical subchannels. FIG. 1B shows 6 logical subchannel groups for an exemplary IEEE 802.16 DL subframe.
Segment. A segment is a set of logical subchannel groups; it is a subdivision of the set of available OFDMA logical subchannels that may include all available logical subchannels. FIG. 1B shows three segments within a DL subframe. Segments are typically associated with the three sectors of a base station cell.
Zones. An IEEE 802.16 OFDMA frame is typically comprised of one or more zones (see FIG. 1C). A zone is one complete logical time-frequency part or partition of an OFDMA subframe. There are a number of types of zones. The most used zone types are called Fully Used Subchannel (FUSC) zones and Partially Used Subchannel (PUSC) zones. FUSC zones use all OFDMA logical subchannels; PUSC zones use a subset of available logical subchannels. A PUSC zone must occur as the first DL zone in every DL subframe. FIG. 1C shows an exemplary PUSC zone 121 within a DL subframe. In addition to FUSC and PUSC zones there are other types of DL and UL zones such as PUSC with the complete set of logical subchannels, optional FUSC, AMC (Adaptive Modulation and Coding), AAS (Adaptive Antenna System), TUSC1 (Tile Usage of Sub-Channel 1) and TUSC2 (Tile Usage of Sub-Channel 2). The maximum number of the various DL zones types within a subframe is 8. The transition between zones is indicated by information elements within frame control structures.
Bursts. A burst is a rectangular or square area within a subframe zone comprised of a specified number of logical subchannels and a specified number of OFDMA symbols. Burst are shown in FIGS. 1A, 1B, 2A and 2B. Bursts contain OFDMA user's FEC encoded and modulated MAC packet or protocol data units. OFDMA supports adaptive burst profiling meaning coding and modulation may be changed for each burst. For each user, the maximum number of bursts to decode in one DL subframe is 64. As shown in FIG. 2B, the location of a burst within a subframe is specified using four fixed-length parameters or fields: OFDMA symbol offset 210, logical subchannel offset 211, number of logical subchannels 212, and number of OFDMA symbols 213.
Slots. Each burst is comprised of a number of slots (see FIG. 2B). A slot is the minimum possible data allocation unit within a OFDMA frame structure, its size is defined in terms of time and frequency. A slot's size depends on zone type and is one logical subchannel by one, two, three, or six OFDMA symbols.
Preamble. A BPSK modulated preamble (see FIG. 2A) is broadcast to all network users and is typically used for DL synchronization (cell search, frame timing acquisition, frequency offset estimation, symbol timing estimation) and channel estimation. It is a reference signal known by all network receivers.
Frame Control Header. A Frame Control Header (FCH) follows the preamble field and for high reliability it is transmitted using QPSK modulation, rate ½ channel coding and four-fold repetition encoding (four logical subchannels with successive logical subchannel numbers). The FCH provides frame configuration information such as logical subchannel groups to be used on the first DL-PUSC zone and a DL Frame Prefix (DLFP). The DLFP provides information for the subsequent DL-MAP (Medium Access Protocol). The DLFP contains information such as subchannel groups used by the DL-MAP, subchannel repetition coding used in the DL-MAP, modulation and channel coding used in DL-MAP, and the length of the DL-MAP burst in units of slots.
DL-MAP and UL-MAP. Referring to FIG. 2A DL/UL MAPs are used to broadcast frame allocation information and other control information for subsequent DL and UL subframes. Allocations specified by the DL-MAP or UL-MAP do not span over multiple zones. DL/UL MAPs begin with header fields followed by a sequence of one or more information elements.
Information Elements. Referring to FIG. 2A DL/UL MAPs contain one information element (IE) for each burst allocated or scheduled for subsequent DL and UL subframes. In both DL/UL MAPs the very last IE is empty and indicates the end of a subframe (not shown in FIG. 2A). Each IE includes a DL or UL interval usage code (DNC or UIUC). An interval usage code specifies a unique burst profile that is to be used for a DL or UL burst. Burst profiles specify a burst's coding and modulation. Each IE also includes fields that define the size and location of its associated burst within a DL or UL subframe. A burst's size and location is represented using four IE fields. The four fields specify a burst's OFDMA symbol offset, the number of OFDMA symbols comprising the burst, its OFDMA subchannel offset, and the number of logical subchannels comprising the burst. The symbol offset field uses the preamble as a reference. Burst power level and repetition coding scheme may also be included within an IE.
Logical Ranging, CQI and ACK Subchannels. Referring to FIG. 2A an UL Ranging Subchannel is used to perform closed-loop time, frequency, and power adjustment as well as bandwidth requests. Four types of ranging are defined: (1) ranging for UL synchronization in time and frequency, (2) initial ranging for when a user enters the network, (3) periodic ranging after a connection is set up, (4) hand-over ranging, and (5) bandwidth request. The UL CQICH (channel quality indicator subchannel) is used by users to feedback channel-state information. The UL ACK Subchannel is used by users to feedback DL Hybrid-ARQ acknowledgements.
The above described OFDMA frame structure requires a significant amount of frame overhead to schedule or allocate bursts to a number of OFDMA users. The majority of the overhead is associated with the DL/UL MAPs. More specifically, within the DL/UL MAPs sequences of information elements are needed to define dynamic DL and UL bursts within subsequent DL and UL subframes. For each IE, the following four fields are required to specify the size and location of a burst within a subframe:                OFDMA_Symbol_Offset        Number_of_Symbols        OFDMA_Subchannel_Offset        Number_of_SubchannelsIndeed, the lengths of these four fields should be minimal in order to reduce frame overhead. To better see the impact of these four fields on frame overhead refer to FIG. 2B where an example DL burst within a subframe's time-frequency plane is illustrated. The units on the time axis are OFDMA symbols. The units on the frequency axis are logical subchannels. It is easy to see from FIG. 2B that the total number of bits required to reference a single burst within the subframe is 27 bits (summation of the number of bits for all four fields). The maximum number of bursts to decode in one DL subframe is 64, hence, the maximum IE overhead required to specify the size and locations of these bursts is 64×27=1728 bits. If the number of OFDMA subcarriers is NFFT=1024 and each OFDMA subchannel carries two bits (assuming QPSK modulation) the four IE fields for burst size and location would require 864 subcarriers for their transmission. This will leave only 160 OFDMA subcarriers to use for the FCH and UL-MAP, note that UL capacity will be significantly impacted. Another OFDMA symbol may be needed for the UL-MAP which would decrease the throughput data rate. In addition, subchannels will be needed for guard-bands, DIUC, and other DL fields that may carry information such as base station identifier, MIMO parameters, multi-hop relaying information, power levels, etc. Repetition coding of the subframe header fields over two or more subchannels will further decrease the number of bits available.        